Structures of Native and Regenerated Celluloses

The crystal structures of native and regenerated celluloses have been determined using x-ray diffraction and least squares refinement techniques. Both structures have monoclinic unit cells containing sections of two chains with 21 screw axes. Models containing both parallel and antiparallel chains were refined in each case by comparison with the x-ray intensities for Valonia cellulose I and rayon cellulose II. For native cellulose, the results show a preference for a system of parallel chains (i.e. all the chains have the same sense). The refinement orients the -CH2OH groups close to the tg conformation such that an 0(6)···H-0(2') intramolecular hydrogen bond is formed. The structure also contains an 0(3)-H···0(3) intermolecular bond along the a axis. All these bonds lie in the 020 plane, and the native structure is an array of staggered hydrogen bonded sheets. In contrast, for regenerated cellulose the only acceptable structure contains antiparallel chains (i.e. the chains have alternating sense). The CH2OH groups of the corner chain are oriented close to the gt position; those of the center chain are close to the tg position. Both center and corner chains have the 0(3)-H···0(5') intramolecular bond and the center chain also has an 0(2')-H···0(6) intramolecular bond. Intermolecular hydrogen bonding occurs along the 020 planes: 0-(6)-H···0(2) bonds for the corner chains and 0(6)-H···0(3) bonds for the center chains, and also along the 110 planes, with a hydrogen bond between 0(2)-H of the corner chain and 0(2') of the center chain. The major consequence of these structures is that native cellulose is seen as extended chain polymer single crytals. The cellulose II structure is compatible with regular chain folding, although there is no direct evidence for such folding.